結合鎳粉放電加工與電流變拋光之製程研發

Abstract

在過去半個多世紀以來，放電加工（Electrical Discharge Machining, EDM）克服了傳統切削難以進行成型的材料，在模具製造業是十分重要的一環。然而，在電能與熱能的作用之下，放電加工後的表面常常會產生再鑄層和放電坑，使表面受到影響。本研究欲將金屬粉末放電與電流變拋光做結合，使放電加工成為一種從粗加工到精加工可以一氣呵成的製程，而複合化的關鍵在於加工液的選擇和間隙的操作使用。首先於固定間隙、不同電場下，觀察加工液中粉末的動態；接著透過間隙的調控，了解加工液的電流變效應最大間隙、放電最大間隙，並探討高速旋轉下的抗離心強度。最後進行SKD11的表面拋光，選用載液黏度、粉末種類、電極尺寸、電壓大小、間隙寬度、粉末混合比例等多種參數，對高硬度的SKD11進行拋光，探討加工前後的效益。 以純電流變液而言，實驗結果指出，在電壓300V的情況下，間隙大於300μm時的電流變效應不夠強，經過一小時的拋光也幾乎沒有表面粗糙度的改善；間隙100μm和300μm之間，為純粹的電流變拋光；間隙50μm到100μm之間，電流變效應伴隨放電現象，工件表面上出現微小放電點，但整體而言有變亮和變平的趨勢。而對於添加金屬粉末於電流變液而言，建議使用低黏度載液，以3:1的比例混入澱粉和鎳粉，在電流變鏈狀結構的輔助下，在50μm可以有均勻的粉末放電行為，而在120~150μm則又可進行純電流變拋光。

In the past half century, EDM（Electrical Discharge Machining）has overcome the difficulty of shaping hard-cutting materials, which traditional cutting methods could not accomplish. Hence, it plays a vital role in manufacturing. However, owing to electrical and thermal effect, craters and recast layers often appear on surfaces after machining. In this study, we try to combine powder-EDM and electro-rheological fluid, making EDM to become a much more mature machining, and the key points are dielectric selection and gap controlling. First, behaviors of different powders between a fixed gap are observerd by various voltage. Second, the ER effect critical gap and discharging maximum gap are obtained by controlling gap sizes. Furthermore, critical rotation speed is also discussed. Last, many factors are chosed to proceed SKD11 surface polishing, such as viscosity, powder species and mixed ratios, electrode sizes, voltage values, gap depths. Experiment underwent under 300V. The result shows that ER effect isn’t strong enough when the gap is larger than 300μm, even though the machining time is 1hr, there’s almost no Ra improvement. When the gap is between 100μm and 300μm, it presents an pure ER polishing effect. When the gap is between 50μm and 100μm, ER effect accompanies with discharge, micro discharging points occured on workpiece, but the surface is still lighter and smoother. In all, low viscosity working fluid and mixed ratio 1:1 of starch and Ni are recommanded, the powder EDM could be carried out uniformly when gap is 50μm, and polishing process could be conducted while gap between 120μm and 150μm.